Modeling iron enrichment in hot-dip galvanneal coatings on interstitial-free steels
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evolution of the phases, t~.21measured the iron content of the coating, and evaluated its powdering properties as a function of annealing time. The point that makes their work particularly interesting is that they have described the development of the interfacial layer, most likely gamma~ phase, tjj as a function of annealing time. They have shown that the interfacial layer growth had three stages. There was an initial rapid growth stage, stage 1, that lasted a few seconds in which the interfacial layer reached a thickness of about 1 /xm. This was followed by a no growth period, stage 2, which lasted about 60 seconds. In stage 2, while the thickness of the interfacial layer remained constant, eta and/or zeta phases were consumed by the growth of the delta phase. Subsequent iron enrichment of the delta phase took place up to approximately 12 wt pct, which roughly corresponds to the iron solubility limit of the delta phase. At this point, stage 3 began and the interfacial layer started to grow again. Stage 2 is probably the stage of greatest practical relevance. As an example, an important coating property, the degree of powdering, undergoes a significant change during the period when the interfacial layer does not grow (Figure 9 of JGM). In order to describe stage 2 kinetics, a simple model can be proposed. It is suggested here that the rate of increase in the coating iron content, d W / d t , is proportional to the difference between a certain saturation coating iron content (in wt pct), Ws, and the coating iron content W (also in wt pct): dW dt
Modeling Iron Enrichment in Hot-Dip Galvanneal Coatings on Interstitial-Free Steels P.R. RIOS Galvannealed coatings are produced by annealing the hot-dip galvanized sheet soon after it emerges from the zinc bath. The zinc coating is then enriched in iron with concurrent development of several phases.I~.2,31 Coating properties depend on its iron content, and in the final galvannealed product, the alloyed Fe-Zn coating contains about 10 wt pct iron. It is therefore of considerable practical interest to describe the kinetics of iron enrichment of the zinc coating. In the present work, a simple quantitative model is proposed to describe the kinetics of iron enrichment of the FeZn coating. Simulation of the galvannealing process has been carried out by Jordan, Goggins, and Marder (JGM) Ill in a hot-dip galvanized titanium-stabilized interstitial-free (IF) steel. In order to simplify the analysis, they held all process variables constant except for the postcoating annealing temperature and hold time. They thoroughly described the
P.R. RIOS, Professor, is with the Escola de Engenharia Industrial Metal~rgica, Universidade Federal Fluminense, Av. dos Trabalhadores 420, Volta Redonda, RJ, 27260-740, Brasil. Manuscript submitted October 6, 1995. 1132--VOLUME 27A, APRIL 1996
= k(ve, -
w)
[11
Equation [1] means that in the beginning of stage 2, when the coating iron content is still low, the reaction is fast. However, as delta becomes saturated in iron, W approaches W,, the reacti
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